Polyester films typified by polyethylene terephthalate have excellent physical and chemical properties and thus are used in various fields such as graphic art, display, wrapping, base film for magnetic recording media, dielectric materials for condenser, etc.
In order to manufacture polyester films having excellent properties, it is essential to impart good running property to the film in the aspects of passing through the manufacturing steps, finishing steps after the film is coated by application or vapor deposition or handling of the product per se. Polyester films, however, are not entirely satisfactory in this respect. The deficiency is, in most cases, manifested by friction and abrasion caused when the film slides in contact with apparatus parts at high speed.
It is well known to properly roughen the surface of films in order to improve their running property and abrasion resistance. In order to achieve this object, fine particles are incorporated in the polyester. Although this is practically employed in industry to some degree, it is not always successful in providing the desired properties.
For instance, when the so-called precipitated particles, which are formed from the catalyst residue during the preparation of polyester, are employed, control of the amount of the particles, particle diameter, prevention of formation of coarse particles, etc. are difficult. Such particles are easily broken when the film is drawn (stretched) and do not bring about good running property and abrasion resistance. Also recycled use of such films is difficult.
Another means is to add inorganic particles of calcium carbonate, titanium dioxide, calcium phosphate, etc., which are inert to polyester. These particles are not subject to breakdown or deformation during the drawing and form rather steep protrusions and, therefore, bring about good running property. However, these particles have poor affinity with polyester and void spaces are formed around the particles when the film is drawn, which markedly degrades the transparency of the film. Also, these particles easily come off from the film surface and become white dust.
It is known to use silica particles which have relatively good affinity with polyester (Japanese Laying-Open Patent Publication No.37-12150 and 53-453696, for instance). As described in Japanese Laying Open Patent Publication No. Sho 43-23960, however, the dispersibility of silica particles in polyester is very poor and, therefore, they coagulate during the synthesis of polyester causing formation of coarse protrusions on the surface of the resulting film and degrading the transparency of the film. Further there is some difficulty that silica reduces the polymerization rate and degrades the thermal stability of the resulting film. The reason for this is not entirely clear but it is surmised that silanol groups existing on the surface of silica particles might react with the metal compound as the polymerization catalyst and the metal compound might be partially trapped by the silica.
We conducted intensive research in order to obtain a film provided with excellent surface properties, running property and other characteristics required for a base film and found that such a film can be obtained by incorporating silica particles which have specific physical properties and completed this invention.